a) Field of the Invention
The present invention relates to coating techniques of an electrode sheet of a non-aqueous cell, capable of improving productivity, providing a high discharge potential, and stabilizing a cell life time.
b) Description of the Related Art
In the course of developments on electronic apparatuses using secondary cells as their power sources, demands for a high capacity of secondary cells are rapidly expanding. To meet the requirement of high capacity, lithium secondary cells are being developed in place of nickel cadmium cells and nickel hydrogen cells. As compared to conventional secondary cells, although lithium secondary cells have already a higher capacity, a much higher capacity is still desired. In addition to developments on electrode materials, increasing the amount of electrode materials filled in a cell can has been studied in order to realize high capacity cells. Approaches to increasing the amount of electrode materials include an increased amount of electrode material capable of being coated and a reduced void factor of electrode material through pressing a coated electrode sheet. FIG. 3 is a cross sectional view of a conventional electrode sheet taken along its longitudinal direction. Generally, an electrode sheet has depolarizing mix layers 52a and 52b formed on both sides of a current collector 51 by coating liquid containing electrode material (hereinafter called depolarizing mix coating liquid). The total thickness of the depolarizing mix layers 52a and 52b is greater than that of the current collector 51, and the void factor of the depolarizing mix layers 52a and 52b after pressing is controlled in a range of 10% to 30%.
Conventional electrode sheets are, however, associated with some problems during manufacture, such as breaking and cutting off electrode sheets at a pressing process. At this pressing process, while an electrode sheet is transported by press rollers in the direction indicated by an arrow 53, a portion of the current collector 51 not coated with electrode depolarizing mix is pressed and then a portion of the current collector 51 covered with the depolarizing mix layers 52a and 52b is pressed. The thickness of the electrode sheet to be pressed changes from that of only the current collector 51 to that of all the current collector 51 and depolarizing mix layers 52a and 52b. This thickness change is acute and severe to such an extent that the current collector 51 is broken at a thickness change portion 54.
With such a steep thickness change, it is difficult to form a uniform circle at the winding start portion (portion of the current collector 51 not covered with depolarizing mix layers) of the electrode sheet and a separator wound together at a cell assembly process. Therefore, the final coiled assembly of an electrode sheet and a separator becomes likely to have a deformed cross section.